At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available to use wireless transfer of data and more applications became available that operate based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users.
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radio communications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e. the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
Modern wireless communication systems targeted for packet-based communications often include multiple-input-multiple-output (MIMO) antenna array configurations. The use of multiple antennas at the transmitter and/or the receiver side can significantly boost the performance of a wireless system. Such MIMO arrays of antennas have the potential of both improving data rates as well as increasing the diversity. The antennas in a MIMO configuration can be located relatively far from each other, typically implying a relatively low mutual correlation. Alternatively, the antennas can be located relatively close to each other, typically implying a high mutual correlation. Which correlation is desirable depends on what is to be achieved with the multi-antenna configuration. i.e., diversity, beamforming, or spatial multiplexing. The MIMO antenna configurations have multiple radio channels that are subject to some degree of frequency selectivity, implying that the channel quality will vary in the frequency domain. The frequency diversity increases a corruption of a transmitted signal in wider-band transmission. In this respect, time dispersion occurs when the transmitted signal propagates to the receiver via multiple paths with different delays as shown for example in FIG. 1, in which a signal S propagates from a node 10 to a user terminal 12 via two paths 14 and 16. The user terminal may be any device that is capable to communicate wireless with the base station. For example, the user terminal may be a mobile phone. In the frequency domain, a time-dispersive channel corresponds to a non-constant channel frequency response as shown in FIG. 2. The radio-channel frequency selectivity corrupts the frequency-domain structure of the transmitted signal and leads to higher error rates for given signal-to-noise/interference ratios. Each radio channel in the MIMO configuration is subject to frequency selectivity, at least to some extent. The extent to which the frequency selectivity impacts the radio communication depends on the bandwidth of the transmitted signal. It also depends on the environment.
Frequency-selectivity may not only be due to the propagation conditions. The radio frequency (RF) chains, including transmit filters, antenna cables and antennas at the base station, are likely to also contribute to the overall frequency variations of the channel unless specific measures are taken to mitigate this kind of impairment by some form of calibration. One type of impairment in this category is time misalignment among the signals received at the user terminal from different antennas of the base station equipped with MIMO antennas. Even a small time difference of the received signals can have a large impact on the effective channel response since a substantial phase difference, linear in frequency, is induced by the time misalignment.
To illustrate the impact of time-misalignment, consider the requirements in wideband code division multiple access (WCDMA), which stipulate that the time difference between the two antennas must be less than 65 ns. Then, the relative phase difference between the two antennas would be on the order of 470 degrees (360×65×1e−9×20e6) for a 20 MHz system. Assuming a three-bit codebook of discrete Fourier transform (DFT) based beamforming vectors, the phase shift between two consecutive beamforming vectors is 45 degrees. This frequency-selectivity would thus alone force the use of roughly 470/45=10 beamforming elements across the bandwidth in order to limit the losses due to ill-matched beamforming elements. As would be discussed later, this would increase the signaling overhead. Thus, the frequency selectivity should be corrected. Prior to discussing existing methods for correcting frequency selectivity. WCDMA and LTE frequency selectivity is discussed next.
In case of a single wideband carrier, such as a WCDMA carrier, each modulation signal is transmitted over the entire signal bandwidth. Thus, in case of the transmission of a single wideband carrier over a high frequency-selective channel as shown in FIG. 3, each modulation symbol will be transmitted both over frequency bands with relatively high quality (A) and frequency bands with low quality (B). Such transmission of information over multiple frequency bands with different instantaneous channel quality is also referred to as frequency diversity. Frequency diversity is desirable in order to improve the quality of the received signal. Thus, the WCDMA system has a good error-rate performance over a frequency-selective channel by its structure.
On the contrary, in case of OFDM transmission that is used in LTE systems, each modulation symbol is confined to a relatively narrow bandwidth and thus, certain modulation symbols may be fully confined to a frequency band with low instantaneous signal strength B, as illustrated in FIG. 4. Thus, the individual modulation symbols will typically not experience any substantial frequency diversity even if the channel is highly frequency selective over the overall OFDM transmission bandwidth. As a consequence, the basic error-rate performance of OFDM transmission over a frequency-selective channel is poorer than the basic error rate in case of a single wide band carrier.
One way to guard against deficiencies (e.g., frequency diversity) in the channel and to improve error rate performance is to premultiply, at the transmitting unit, the transmission data streams by a precoding matrix, chosen based on channel information. This precoding technique improves the performance of a MIMO system by transforming the information carrying transmit vector so that it better fits the channel conditions. Precoding can been used based on knowledge of the full channel state information of the transmitting unit. The precoding can be performed based on instantaneous channel information, without channel information, or some combination thereof. The precoding may be implemented by performing a linear transformation on the information carrying vector prior to transmission. Such a linear transformation is usually represented by a matrix. Precoding is used as part of WCDMA and is likely to be a part of LTE as well.
There are two basic forms of precoding presently used in telecommunication systems, codebook based and non-codebook based. Codebook based precoding implies that the precoding matrix implementing the linear transformation is selected from a countable and typically finite set of candidate matrices. The transmit precoder is chosen by the receiver from a codebook of precoding matrices known to both the receiver and the transmitter. The mentioned set constitutes the codebook. Channel dependent codebook based precoding is similar to channel quantization because a set of channel realizations map to a certain precoding element. Non-codebook based precoding, on the other hand, does not involve any quantization, and the precoding element can thus, for example, be a continuous function of the channel matrix. A special case of precoding is beamforming. In beamforming, a single information carrying symbol stream is multiplied by a channel dependent vector, which adjusts the phase of the signal on each transmit antenna, so that coherent addition of the transmit signals can be obtained at the receiver side. The beamforming method provides diversity as well as increases the SNR.
The precoder element to be used for the transmission of data to the user terminal may need to be signaled, by way of feedback signaling and/or signaling of the chosen precoder element in forward link, i.e., in a direction from a base station to the user terminal. The term “base station” is used in this disclosure as a generic term for the “NodeB” of the WCDMA system, the “eNodeB” of the LTE system, and other nodes of other systems as will be appreciated by those skilled in the art. The feedback signaling is one way for the receiver to provide channel information to the transmitter.
Several different approaches are known for forward link signaling associated with precoding. One approach is explicitly signaling the precoder element index in forward link. Another approach is implicitly signaling the precoder element index using precoded reference symbols/pilots, which together with non-precoded reference symbols can be used at the receiver side to determine the used precoder element. Still another approach is to use precoded reference symbols also for the demodulation of the data, so-called dedicated reference symbols, and to incorporate, from the receiver's point of view, the precoder element into the effective channel. For maximum performance, the precoding element may be chosen to match the effective channel, including transmit and receive filters, channel responses of antenna cables, and the actual propagation channel. If the effective channel varies, as discussed above with regard to the frequency selectivity, over the bandwidth allocated to communication, it is preferable to adapt the precoding over frequency as well, in order to obtain a better match with the frequency-selective channel. However, this process affects the signaling of the precoder elements and a finer frequency granularity of the feedback and forward link signaling may be needed. If dedicated reference symbols are used for the process of matching the precoding element with the effective channel, it results in a reduced coherence bandwidth of the effective channel, which means that the channel estimation procedures at the receiver side may have less data to average over and thus, negatively affects the estimation accuracy. Therefore, channel-dependent precoding has the potential of providing performance gains i.e., correcting the frequency selectivity, and it is the preferred precoding to correct the corruption of transmitted signals. However, the precoding requires certain conditions to be satisfied, as discussed next.
Achieving the gains of the channel-dependent precoding relies on, for example, the ability of the precoder element to closely match the effective transmission channel. Because the effective channels are frequency-selective, traditional telecommunication devices and systems require a calibration process to constantly be performed by the base station since the used precoder element needs to track the channel as the channel varies over the frequency. The propagation part of the effective channel is time-varying and can be challenging to compensate for without incurring additional signaling overhead, which is undesirable. Also, differences in cable lengths over which signals can be conveyed, e.g., between nodes within the fixed parts of the network, imply that the transmitted signals from the antennas may not be time-aligned. In addition, the transmit filters provided in base stations transmit chains may be frequency-selective and moreover, may have different properties depending upon a connection between a particular filter and an antenna connected to the filter. All these factors increase the variations over frequency of the effective channel and thus, contribute to the previously mentioned problems in the traditional communication systems.
It is one object of the next exemplary embodiments to overcome these and other problems with respect to communication systems.